A new method for retrieving equivalent cloud base height and equivalent emissivity by using the ground-based Atmospheric Emitted Radiance Interferometer (AERI)

Abstract

In the paper, we propose a new method of identifying the clear sky based on the Atmospheric Emitted Radiance Interferometer (AERI). Using the Atmospheric Radiation Measurement (ARM) Mobile Facility (AFM) dataset in Shouxian in 2008, we simulate the downwelling radiances on the surface in the 8–12 μm window region using Line-By-Line Radiative Transfer Model (LBLRTM), and compare the results with the AERI radiances. The differences larger (smaller) than 3 mW (cm2 sr cm−1)−1 suggest a cloudy (clear) sky. Meanwhile, we develop the new algorithms for retrieving the zenith equivalent cloud base height (CBHe) and the equivalent emissivity (ɛe), respectively. The retrieval methods are described as follows. (1) An infinitely thin and isothermal blackbody cloud is simulated by the LBLRTM. The cloud base height (H) is adjusted iteratively to satisfy the situation that the contribution of the blackbody to the downwelling radiance is equal to that of realistic cloud. The final H is considered as CBHe. The retrieval results indicate that the differences between the CBHe and observational cloud base height (CBH) are much smaller for thick low cloud, and increase with the increasing CBH. (2) An infinitely thin and isothermal gray body cloud is simulated by the LBLRTM, with the CBH specified as the observed value. The cloud base emissivity (ɛc) is adjusted iteratively until the contribution of the gray body to the downwelling radiance is the same as that of realistic cloud. The corresponding ɛc is ɛe. The average ɛe for the low, middle, and high cloud is 0.967, 0.781, and 0.616 for the 50 cases, respectively. It decreases with the increasing CBH. The retrieval results will be useful for studying the role of cloud in the radiation budget in the window region and cloud parameterizations in the climate model.